1. Field of the Invention
This invention relates to an evaporative fuel-processing system for internal combustion engines for vehicles, and more particularly to an evaporative fuel-processing system which has a function of detecting abnormalities in an evaporative emission control system of the engine.
2. Prior Art
Conventionally, there has been widely used an evaporative fuel-processing system for internal combustion engines for automotive vehicles, which comprises an evaporative emission control system having a canister with an air inlet port provided therein, a first control valve arranged across an evaporative fuel-guiding passage extending between a fuel tank of the engine and the canister, and a second control valve arranged across a purging passage extending between the canister and an intake system of the engine.
An evaporative emission control system of this kind temporarily stores evaporative fuel in the canister, and then purges the evaporative fuel into the intake system of the engine.
Whether an evaporative emission control system of this kind is normally operating can be checked, for example, by bringing the evaporative emission control system into a predetermined negatively pressurized state, measuring a change in the pressure within the fuel tank (tank internal pressure) occurring with the lapse of time after the evaporating emission control system has been brought into the predetermined negatively pressurized state, by a tank internal pressure sensor which detects the tank internal pressure, and determining whether the system is normally operating, from the measured tank internal pressure, as proposed by Japanese Patent Application No. 3(1991)-262857 and corresponding U.S. Ser. No. 07/942,875 assigned to the assignee of the present application, for example.
According to the method of the earlier application, an amount of change in pressure prevailing within the evaporative emission control system is detected by the tank internal pressure sensor, to determine an abnormality in the system in such a manner that if the detected pressure change amount is below a predetermined value, it is presumed that an amount of evaporative fuel leaking from the system to the outside is small and hence it is determined that the system is normally functioning, whereas if the detected pressure change amount exceeds the predetermined value, it is presumed that evaporative fuel is leaking in a large amount from the system to the outside, and hence it is determined that the system is malfunctioning.
The abnormality detecting method according to the earlier application still has room for further improvement, as follows:
The upper surface of fuel within the fuel tank largely moves or stirs when the vehicle is in a particular running condition such as acceleration, deceleration and turning. Consequently, the pressure within the fuel tank largely changes when the vehicle is in such a particular running condition. Besides, when the upper surface of fuel within the fuel tank largely moves, evaporative fuel can be generated in large amounts.
Further, the amount of evaporative fuel generated within the fuel tank can vary depending upon temperature within the fuel tank and an amount of fuel stored in the fuel tank.
A change in the pressure within the fuel tank and generation of a large amount of evaporative fuel in the fuel tank affects the rate of change in the tank internal pressure which occurs with the lapse of time after the evaporative emission control system has been negatively pressurized, which causes an error in the abnormality determination.
However, in the above-mentioned abnormality detecting method according to the earlier application, determination of the abnormality of the evaporative emission control system is made based upon the amount of change in the tank internal pressure or the amount of evaporative fuel generated within the fuel tank. Therefore, when the pressure within the fuel tank largely changes or when a large amount of evaporative fuel is generated in the fuel tank, due to the running of the vehicle in such a particular running condition as mentioned above, etc., it can be erroneously determined that the system is abnormal even when it is functioning normally.